some portion of the population has intakes above the UL, a risk may exist and the need to take action to reduce population intakes should be evaluated. For example, the UL for niacin for adults is 35 mg/day. The LOAEL for niacin is 50 mg/day and the uncertainty factor is 1.5 (indicating a good level of confidence in the data). The adverse effect noted is a relatively benign vasodilation causing flushing of the skin that may be accompanied by a burning, itching, or tingling sensation; this effect is readily reversible by a reduction in intake. The UL for vitamin B6 is 100 mg/day for adults and the NOAEL is 200 mg/day with a UF of 2. The adverse effect observed—sensory neuropathy—is a serious and irreversible condition. Therefore, public health concern over a segment of the population routinely consuming niacin in excess of the UL would not be as great as if a segment of the population were routinely consuming vitamin B6 in excess of the UL.
Figure 6-3 illustrates a situation in which usual dietary intake from foods represents no discernible risk but the addition of intakes from supplement usage makes a fraction of the population potentially at risk. Figure 6-4 represents the type of analysis that would apply when